Aromatic polyisocyanates have been employed, indirectly, as cross-linkers for polyepoxides. (See U.S. Pat. No. 3,687,897.) The resulting cross-links comprise oxazolone moieties which are theoretically derivable by direct oxirane/isocyanate adduction but cannot in fact be readily so formed, even in the presence of epoxide curing catalysts. However, when the isocyanate is adducted with an alcohol, the resulting carbamate will react, at temperatures of 130.degree.-155.degree. C. and in the presence of certain phosphonium catalysts, with oxiranes, to split out the alcohol and form oxazolidinone ("oxazolone") groups. That is, the reaction of alcohol "blocked" NCO groups with oxirane groups can be catalyzed at temperatures below those (180.degree. C., for example) at which such blocked isocyanates, per se, deblock.
However, the foregoing use of blocked isocyanates poses the problem of what to do with the liberated alcohol (meOH, typically). Such alcohols are monofunctional (otherwise polymerization with the isocyanate is likely) and cannot function as cross-linkers. Consequently, the agent must be removed or, at best, functions as a reactive diluent or a generally unwanted plasticizer.
Another problem with the foregoing use of blocked aromatic isocyanates is that of premature "deblocking", which results in gelling at temperatures high enough to effectively reduce epoxy resin viscosities. That is, the carbamate reacts at lower temperatures than is desirable; the epoxy/blocked isocyanate system is not adequately latent for some important applications.
Carbamate groups formed from aliphatic isocyanates are less oxirane-reactive, so much so, in fact, as to be excessively latent. Also, the problem of the liberated alcohol would not be avoided.
For purposes other than epoxide modification or curing, polyisocyanates are commonly blocked with not only alcohols but other --OH or .dbd.NH functional compounds, such as phenols, oximes and lactams. Phenols are oxirane reactive but phenol/isocyanate adducts deblock at even lower temperatures than the alcohol adducts. The other conventional types of blocking agents are not adequately reactive with oxirane groups and must be removed or left as an adulterant in the cured product.
Thus, a blocking agent which will not polymerize with the polyisocyanate, does not dissociate from it prematurely and can be transformed at curing temperatures into an epoxide cross-linker--though highly desirable--is not contemplated in the prior art.